<!DOCTYPE html>
<html lang="zh-CN">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>三栏布局</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        html,body{
            height: 100%;
        }
        .top,.bottom{
            height: 50px;
            background: gray;
        }
        .middle{
            background: yellow;
            height: calc(100% - 100px);
        }
        .left{
            width: 200px;
            background: red;
            float: left;
            height: 100%;
        }
        .center{
            background: rgb(7, 231, 194);
            margin-left: 200px;
            margin-right: 200px;
            height: 100%;
        }
        .right{
            background: green;
            width: 200px;
            float: right;
            height: 100%;
        }
        .middle::after{
            content: "";
            display: block;
            clear: both;
        }
    </style>
</head>
<body>
    <div class="top">上</div>
    <div class="middle">
        <div class="left"></div>
        <div class="right"></div>
        <div class="center"></div>
    </div>
    <div class="bottom">下</div>
</body>
</html>